1. Field of the Invention
The present invention relates to an endoscope apparatus, more particularly, a constitution used in medical fields for forming and displaying a spectral image (video) made up of image information of arbitrarily selected wavelength ranges.
2. Description of the Related Art
Recently, in an electronic endoscope apparatus which uses a solid imaging device, spectral imaging combined with a narrow band pass filter on the basis of a spectral reflectance in the alimentary canal (gastric mucosa and the like), namely, a narrow band filter built-in an electronic endoscope apparatus (Narrow Band Imaging-NBI) has become the focus of attention. In place of rotational filters of R (red), G (green) and B (blue) by a frame sequential method, this system is provided with band pass filters of three narrow bands (wavelengths), outputs sequentially illumination light via these narrow bandpass filters, and conducts processing the same as in the case of red (R), green (G) and blue (B) signals while changing respective weightings to three signals obtained from these illumination lights, thereby forming a spectral image. This spectral image is able to realize microstructures and the like in gastrointestinal tracts such as the stomach and large intestine, which would otherwise not be realized.
In contrast, unlike the frame sequential method using the above-described narrow band pass filters, as described in Japanese Published Unexamined Patent Application No. 2003-93336, it has been proposed that in the simultaneous method in which micro-mosaic color filters are arranged on a solid imaging device, a spectral image is formed by the computing process on the basis of image signals obtained from white light. In this method, the relationship between numeric data of the respective R, G, and B color sensitivity characteristics and numeric data of spectral characteristics of a specific narrow bandpass is determined as matrix data (coefficient sets) and computing is made for the matrix data and the R, G and B signals to obtain spectral image signals artificially via the narrow bandpass filters. Where a spectral image is formed by such computing, it is not necessary to provide a plurality of filters corresponding to desired wavelength ranges and to provide these change-over arrangements, there by successfully avoiding increases in the size of a system and reducing cost.
However, since each wave range of the signals constituting a spectral image is a narrow band on formation of the spectral image in the endoscope apparatus, the image is reduced in brightness as compared with an ordinary image formed by RGB signals, which is a problem. FIG. 5 shows spectral sensitivity characteristics of color filters such as R (red), G (green) and B (blue) used in an elementary color-type CCD, a solid imaging device, and an example of individual wavelength ranges forming the spectral image in the present invention. As shown in this drawing, the spectral image is constituted with signals made up of, for example, wave range λ1 (narrow band centered at 500 nm), wave range λ2 (narrow band centered at 620 nm) and wave range λ3 (narrow band centered at 650 nm) These wavelength ranges of λ1, λ2 and λ3 are reduced, as compared with a RGB wave range, and they are a narrow band. Therefore, all signal components in the wavelength ranges of λ1, λ2 and λ3 are extremely reduced, as compared with all signal components in the RGB wave range, and lacking in brightness components.
Further, in formation of an ordinary image, output signals of a CCD are amplified to result in an increase in noise components, and therefore the noise components of the image signal are also removed. In formation of a spectral image as well, care must be taken so as not to increase noise components when signals of individual wavelength ranges are amplified.
In addition, in a spectral image which has become the focus of attention, microstructures and others can be visualized, which has not conventionally been obtained. If specified microstructures are displayed so as to be remarkably distinguished from other tissues, valuable information on an object to be observed can be provided useful for making a diagnosis and others.